We have studied the production of absolute amounts of Hb A and Hb F in cultures of human BFU-E grown in methyl cellulose culture system. In this setting Hb F production is enhanced as compared to similar cultures grown in plasma clots. A study of the temporal sequences shows that Hb A production reaches a peak later and proceeds longer than Hb F synthesis. This asynchrony correlates with the relative prevalence of erythroid cells at different stages of maturation. We propose to extend these studies: (1) to include BFU-E from normal donors as well as from those with high in vivo rates of Hb F production; (2) to re-examine the effect of adherent cells in the modulation of Hb F/Hb A synthesis, which we have previously demonstrated, in relative to the asynchrony of synthesis. We expect that a complete accounting of these relations may clarify some of the mechanisms for enhanced Hb F synthesis in culture and in vivo.